Ask Ars: What is the best way to use a Li-ion battery?

Ask Ars tackles the best ways to keep Li-ion batteries in shape, from charges …

Question: How do I take care of a Lithium Ion battery to prolong its life? Should I charge it frequently or drain it fully before charging it?

Lithium ion batteries are particular about their operating conditions, and there are a lot of small things that can contribute to better quality of life. Li-ion batteries have a reasonably finite lifespan and can hold only a fraction of their original capacity after a few years, but things like operating temperature, how long the battery spends plugged in, how the battery is used, and the charge cycling you asked about can contribute to how long the battery lasts. If Michael Pollan had to sum up ideal Li-ion battery usage, he might say something like, "Use your battery. Not too much. Mostly for small apps."

One of the worst things you can do to a Li-ion battery is to run it out completely all the time. Full discharges put a lot of strain on the battery, and it's much better practice to do shallow discharges to no lower than 20 percent. In a way, this is like people running for exercise— running a few miles a day is fine, but running a marathon every day is generally not sustainable. If your Li-ion powered device is running out of juice on a daily basis, you're decreasing its overall useful lifespan, and should probably work some charging stations into your day or change your devices' settings so that it's not churning through its battery so quickly.

There used to be certain types of batteries whose "memory" of their total charge capacity seemed to get confused by shallow discharges. This is not, and never was, the case with Li-ion batteries. However, if you are using something like a notebook computer that gives you time estimates of how much longer the battery will last, this clock can be confused by shallow charging intervals. Most manufacturers recommend that you do a full discharge of the battery about once a month to help your device calibrate the time gauge.

One common misconception is that Li-ion batteries will only count charge cycles if the battery is drained completely in one session; another is that the battery counts one charge cycle for every instance the device is unplugged and plugged in again. Neither of these is true—Li-ion batteries actually count charge cycles based on a 100 percent discharge even when it's summed over multiple sessions. For example, if you discharge a battery to 50 percent one day, charge it back to 100 percent, then discharge it 50 percent again the next day, that is counted as one "cycle" of the battery. So shallow discharges, in all these regards, are ideal for a Li-ion battery.

On the other end of the spectrum, keeping a Li-ion battery fully charged is not good for it either. This isn't because Li-ion batteries can get "overcharged" (something that people used to worry about in The Olden Days of portable computers), but a Li-ion battery that doesn't get used will suffer from capacity loss, meaning that it won't be able to hold as much charge and power your gadgets for as long. Extremely shallow discharges of only a couple percent are also not enough to keep a Li-ion battery in practice, so if you're going to pull the plug, let the battery run down for a little bit.

Keep it cool

Another thing that Li-ion batteries hate is heat. This somewhat less of a problem for cell phones, but a big problem for notebooks. Even using a battery at room temperature for a year can bring its capacity down by as much as 20 percent, and the interior of most computers is a mite cozier than than that. So in a unfortunate twist of fate, laptop batteries usually spend the most time in the worst possible state: plugged in at 100 percent charge, running at an elevated temperature.

There's usually not too much you can do about the temperature issue. If you're going to be using a notebook plugged in at a desk for an extended amount of time, possibly running some intensive programs, you can remove the battery to spare it the heat wave. Turning off your computer when you're not using it is a tiny bit helpful. But for any gadget with a Li-ion battery, keep it out of attics, direct sunlight, the tundra, or anywhere there will be extreme temperatures. To the smartphones I've let sit on picnic tables in the sun and the mp3 players I've left in cars all day long: I'm sorry.

Running the battery out very quickly by drawing a lot of power at once is another way to cause it a lot of strain. For example, running a graphics-intensive game on a smartphone or a notebook for a couple of hours while unplugged is worse for the battery than depleting it over several hours while e-mailing or Internet-browsing (heat is a factor here, too). Again with the running analogy: it's probably harder on you to sprint a mile than to jog it.

If you have a spare battery you don't use that often, manufacturers say the best way to store it is at 40 or 50 percent charge in a cool place, like a refrigerator (but not too cold, as extreme temperatures cause capacity loss, so don't put it in the freezer just because there's extra room next to the peas).

These usage points are not going to drastically affect the lifespan of your battery; that is, you can't make it last forever if your carefully cycle the battery down to 50 percent the second it's full and never use it for anything more intensive than the occasional flash game. Most manufacturers will give you an "up to" figure (up to 80 percent of its original capacity after 1,000 cycles, for example), and careful use will help you reach that, but you won't get too far beyond it. However, following these guidelines (Use your battery. Not too much. Mostly for small apps.) will help your reach the upper echelons of manufacturer estimates.

Casey Johnston
Casey Johnston is the former Culture Editor at Ars Technica, and now does the occasional freelance story. She graduated from Columbia University with a degree in Applied Physics. Twitter@caseyjohnston

All of these suggestions also apply to the newest electric cars (Volt, Leaf), not just computers. These rules also explain why the car manufacturers did certain things with their battery management systems.

What about charging a device, but not to 100%? For instance, I use my iPhone with a dock in my car to play music, but my 1/2 hour commute isn't enough to charge the battery to 100%. Is going from 75% to 83% and then letting it discharge again bad for it? Should I just plug in the audio and not charge it in this circumstance?

Actually, it feels like everything is bad for these batteries. Just different degrees of bad.

I have just resigned myself to the fact that a cell phone battery only lives about a year, and a little more than that for a laptop. I would rather use my devices how I need to rather than try and micro-manage battery lifetime.

How long is being plugged in for an extended amount of time? For example, I charge my phone while I sleep and it may finish 4 or 6 hours before I wake up. Is that bad for the battery or should I be good to go?

Lenovo's Thinkpad power management software lets you set charge thresholds for your battery. For example, you can tell it to stop charging at 70% and only start charging if it dips below 30%. I've set my 2.5 year old T400 with those thresholds, and I've still got 100% of the original capacity. I mostly use it on AC power, but I'm pretty sure that if I had left it with the default behavior (always charge to 100%) I'd be down to 70-80% capacity by now.

Great tip for determining if your battery is on it's last legs: spin it!

As Li+ batteries age they tend to swell & though it's not absolute, the general rule is that more swelling means worse capacity. So, if you have a 'square' type battery (like those found in most smartphones) you can gauge how close to end-of-life the battery is by setting it on a flat surface & trying to spin it. If it spins freely then the cell has swelled & it's probably not the best cell.

It can be summed up by thinking of a battery as what it is, a reservoir of milliamp-hours.

If you remove 200 of them, you'll find you can only force around 199 back in, so in time the battery wears. However, it gets a bit more complex than that.

There's a set cut-off minimum a lithium-ion can be allowed to discharge to, it's about 3.0V/cell, of a nominal 3.7V and a fully charged 4.1V. If it discharges below 3.0V per cell, the cell will lose capacity catastrophically as it de-polarises. By 2.0 V/cell, the cell has no appreciable capacity left at all.

So every lithium-ion has a built in hard-cut which triggers at about 3.0V/cell. However, the device you're using also has a soft-cut, usually about 3.4V/cell. This represents about 10% of the cell's total usable capacity which never will be used, but the battery's controller has to plain guess where it is, as the device never allows the battery to hit the 3.0V/cell "zero point". So in time the "0%" point as reported to the device creeps up and creeps up until the battery has very little run-time remaining, even though its actual wear level is quite low.

When this happens, on a laptop (non-Mac, they do not have the ability to run regardless of battery level) you turn off all power management, tell it to "Do nothing" at the low battery and critical battery points and just let it run until it hard-isolates the battery. You might be surprised how long it lasts with 0% battery remaining. Then leave the device off and charge it fully. In one quite hilarious discharge, I ran a netbook for two and half hours on 0% battery, the battery wear level dropped from 68% to 9% on the next charge.

This is a full battery recalibrate and can usually salvage 30 min to one hour of laptop runtime from an older battery.

This is often known as the "digital memory" of the battery, as it stems from slight errors in counting charge accumulating in the battery controller's memory.

What about charging a device, but not to 100%? For instance, I use my iPhone with a dock in my car to play music, but my 1/2 hour commute isn't enough to charge the battery to 100%. Is going from 75% to 83% and then letting it discharge again bad for it? Should I just plug in the audio and not charge it in this circumstance?

Actually, it feels like everything is bad for these batteries. Just different degrees of bad.

The key is avoiding the limits -- both on the full side & the empty side as both high voltage & low voltage conditions result in higher degradation rates. So, as long as you phone is not near 100% when you plug in, you shouldn't be doing an unusual amount of harm.

I've read that Apple mislead their users with their battery advice. Is that true?

The little I knew about Li-ion batteries, I learned from reading the section about batteries in the small manual included with my old Apple Powerbook. And, basically, it said what this article said. In fact, I just checked, and it's obviously simplified, but the article seems to agree with this link: http://www.apple.com/batteries/notebooks.html

Not sure if at any time in the past Apple (or any other manufacturer) has said any lies in relation with maximizing the life of a battery, though.

P.S. Don't let me started with regards to lying about battery life, btw. Apple is one of the few companies who doesn't "lie" incredibly when specifying how long can something last when using just the battery. Years ago I had a Sony Clie PDA that, according to the box, lasted for 15 *days* on battery alone (you should have read the small print at the bottom!).

The key is avoiding the limits -- both on the full side & the empty side as both high voltage & low voltage conditions result in higher degradation rates. So, as long as you phone is not near 100% when you plug in, you shouldn't be doing an unusual amount of harm.

Occasionally you should hit the limits as it allows the battery controller to gain hard evidence as to where those limits are. Otherwise it guesses by counting milliwatts in vs milliwatts out.

It can be summed up by thinking of a battery as what it is, a reservoir of milliamp-hours.

If you remove 200 of them, you'll find you can only force around 199 back in, so in time the battery wears. However, it gets a bit more complex than that.

There's a set cut-off minimum a lithium-ion can be allowed to discharge to, it's about 3.0V/cell, of a nominal 3.7V and a fully charged 4.1V. If it discharges below 3.0V per cell, the cell will lose capacity catastrophically as it de-polarises. By 2.0 V/cell, the cell has no appreciable capacity left at all.

So every lithium-ion has a built in hard-cut which triggers at about 3.0V/cell. However, the device you're using also has a soft-cut, usually about 3.4V/cell. This represents about 10% of the cell's total usable capacity which never will be used, but the battery's controller has to plain guess where it is, as the device never allows the battery to hit the 3.0V/cell "zero point". So in time the "0%" point as reported to the device creeps up and creeps up until the battery has very little run-time remaining, even though its actual wear level is quite low.

When this happens, on a laptop (non-Mac, they do not have the ability to run regardless of battery level) you turn off all power management, tell it to "Do nothing" at the low battery and critical battery points and just let it run until it hard-isolates the battery. You might be surprised how long it lasts with 0% battery remaining. Then leave the device off and charge it fully. In one quite hilarious discharge, I ran a netbook for two and half hours on 0% battery, the battery wear level dropped from 68% to 9% on the next charge.

This is a full battery recalibrate and can usually salvage 30 min to one hour of laptop runtime from an older battery.

This is often known as the "digital memory" of the battery, as it stems from slight errors in counting charge accumulating in the battery controller's memory.

All good points. One small error -- Li+ is typically charged to 4.2V unless the manufacturer is trying to squeeze some extra cycle life out of it (with the additional effect of reducing its capacity).

On the other end of the spectrum, keeping a Li-ion battery fully charged is not good for it either.

This is kind of the frustrating thing about docking a laptop -- I know that keeping it fully charged for long periods of time isn't ideal, but it's not like I'm going to dock and undock the thing frequently just to avoid it. It would be nice if the laptop would charge the battery to some reasonable level -- say 70% -- and then disconnect the circuit entirely, using just dock power, until I undock.

The key is avoiding the limits -- both on the full side & the empty side as both high voltage & low voltage conditions result in higher degradation rates. So, as long as you phone is not near 100% when you plug in, you shouldn't be doing an unusual amount of harm.

Occasionally you should hit the limits as it allows the battery controller to gain hard evidence as to where those limits are. Otherwise it guesses by counting milliwatts in vs milliwatts out.

Yup, to increase the accuracy of capacity gauging you are absolutely correct. I was only focusing on maximizing the cycle life of the cell.

On the other end of the spectrum, keeping a Li-ion battery fully charged is not good for it either.

This is kind of the frustrating thing about docking a laptop -- I know that keeping it fully charged for long periods of time isn't ideal, but it's not like I'm going to dock and undock the thing frequently just to avoid it. It would be nice if the laptop would charge the battery to some reasonable level -- say 70% -- and then disconnect the circuit entirely, using just dock power, until I undock.

I set my laptop to charge the battery only when below 80%. After 600 cycles and 18 months I'm down to 55% of the original capacity. Is that considered typical, or terrible? Both?

Well manufactured cells should give you >80% capacity in 500 cycles at room temperature -- so your results are poor in comparison. However my experience with laptops is that temperature has a huge effect so I'm not surprised.

This is one reason I would never consider an Apple laptop. Embedded batteries just don't make sense to me in the laptop space.

For example, you can tell it to stop charging at 70% and only start charging if it dips below 30%. I've set my 2.5 year old T400 with those thresholds, and I've still got 100% of the original capacity.

But surely if you only ever charge to 70%, you only ever get at most 70% of the battery's lifetime, regardless of how good a condition it's in? What's the point of that extra capacity if you don't use it?

I'm glad that this article was run. There are so many hold-overs from the NiMH and NiCd days that are just plain wrong for Li-ion batteries.

The most common I still hear is for people to charge their electronics for 24+ hours when they first get them, despite Li-ion batteries being shipped with 40% charge or so already.

Also draining your battery all the way. As mentioned in the comments, it can be useful to do every once in a while... but it's not something that you want to do regularly (maybe once every few months at most).

Deep discharge/charge cycles have actually been shown to purge micro-pores in the cathode/anode rods thus restoring a significant amount of current carrying capacity so if you have an application that needs to quickly pull charge it's a good idea to deeply discharge the cells every so often.

Umm hasn't Apple said to drain your batteries to close to 0 as possible (full discharge) if you can? Who is right here. In fact most battery folks (at the battery shop here in Austin TX) say to discharge them fully every once other use (if you can).

Umm hasn't Apple said to drain your batteries to close to 0 as possible (full discharge) if you can? Who is right here. In fact most battery folks (at the battery shop here in Austin TX) say to discharge them fully every once other use (if you can).

This doesn't really help much unless you can actually hit the hard-cutoff 0 point. In a Mac or practically all cellphones, you can't actually do this.

If you're going to be using a notebook plugged in at a desk for an extended amount of time, possibly running some intensive programs, you can remove the battery to spare it the heat wave.

I thought that some laptops wouldn't run properly with the battery removed - something about the peak power draw being more than the AC adapter could supply, so the battery was required. I might be remembering that wrong, though.

Umm hasn't Apple said to drain your batteries to close to 0 as possible (full discharge) if you can? Who is right here. In fact most battery folks (at the battery shop here in Austin TX) say to discharge them fully every once other use (if you can).

There's no benefit to draining a Li-ion battery fully except to re-calibrate the electronics. That shouldn't need to be done more than every couple of months at most. People are full of bullshit about batteries.

Umm hasn't Apple said to drain your batteries to close to 0 as possible (full discharge) if you can? Who is right here. In fact most battery folks (at the battery shop here in Austin TX) say to discharge them fully every once other use (if you can).

not in current model battery use, my old, old powerbook had some voodoo apple suggested doing when you first got it. it was something about draining it, then filling it back up.

my current macbook's battery advice amounted to "fuck it, plug it in do whatever."i had assumed some tech hurdles had been reached.

Lithium Ion don't have memory. You can plug and unplug as much as you want.

I'm not sure what your point is. Lithium ion batteries don't have the memory "trait" that NiCad batteries have, but their overall life _is_ affected by how you actually use them. No rechargeable battery lasts forever.

I have just resigned myself to the fact that a cell phone battery only lives about a year, and a little more than that for a laptop. I would rather use my devices how I need to rather than try and micro-manage battery lifetime.

Xavin, it all depends on the manufacturer in my opinion. I have a 2 year old iPhone with I use everyday and the battery is still nearly as good as when it was new. The only slide in battery life that I see is when I use it for heavy gaming or heavy cell phone use. When it was new, heavy use would decrease the battery life to 3/4 of a day - now heavy use will give me 2/3 of a day. But again, most days I get through with plenty of battery to spare, and I surf, email, text, use the phone, do a little gaming, do much organizing, note taking, and 6+ hours of music and podcast listening on the average day. I plug it into the charger every night about an hour before bed (the battery status usually says between 30 - 40%), and unplug it each morning before heading off to work.

My daughter has a 6 year old iBook G4 laptop that is still going strong, she uses it every day and plugs it in 3-4 times a week. When she is not using it she just closes the lid and it goes to sleep, she rarely powers down or reboots. I have never heard of anyone getting 6 years of life out of a battery with anything but an Apple product. I have a 5 year old iBook G4 at work that when it was new I bought a spare battery for. I have never used the spare battery as I haven't ever run out of juice in the middle of a project. The original iBook battery is still going strong. I think Apple is amazing in their battery technology and energy management.

I understand why most people poo-pooed Apple when they started making their gear without replaceable batteries. Most people have had awful experience with batteries. But those of us who have actually used Apple products on a daily basis know that the non-replaceable batteries were actually a good idea for Apple as they could pack more battery into their products without all the cover plastic and latching mechanisms etc., and Apple batteries/energy management have an excellent track record for me.

Umm hasn't Apple said to drain your batteries to close to 0 as possible (full discharge) if you can? Who is right here. In fact most battery folks (at the battery shop here in Austin TX) say to discharge them fully every once other use (if you can).

Discharging Li-ion batteries that often is definitely harmful. They might be talking about older NiCD batteries (where discharging them all the way was beneficial to prevent "memory").

So driving my Tesla Roadster during the summer in Phoenix is a bad thing? Kind of kills the utility of electric cars for people living in the desert I'd say.

Yes. I used to live in Phoenix & I can assure you that ALL car batteries die more quickly there -- no matter what type. Whatever you do, don't plug in your new electric vehicle when it's in the summer sun. That is pretty much the worst thing you could do for it.